1. Field of the Invention
The present invention relates to video cameras and, more particularly, to a video camera in which an image can be constantly focused even when a picture is taken of an object located within a defocusing area of a zoom lens.
2. Description of the Related Art
Recently, consumer video cameras have become popular because they have become miniaturized and light in weight by using a solid state imaging device, such as a CCD (charge-coupled device), and a small video tape recorder (VTR) recording unit, such as an 8-mm VTR in one body. Further, an optical system of the imaging unit uses a zoom lens so that a picture of an object can be taken under various lens conditions from a wide angle lens mode to a telephoto lens mode by way of a single zoom lens. Therefore, recent consumer video cameras have become very easy to use.
As is well known, the zoom lens is a lens system that can continuously change a focal distance without changing an image point. The zoom lens is fundamentally comprised of a lens group comprised of a focusing lens unit, a variable power or variator lens unit and a fixed lens unit.
The focusing lens unit is fixed when a zoom lens is operated, and moved when the zoom lens is focused. The variable power lens unit changes the focal length by moving a plurality of sets of lenses separately by using cams, for example, and also constantly keeps a focal surface at a predetermined position. The fixed unit corrects aberration generated in the focusing lens unit and the variable power lens unit determines the range of the focal distance.
A ratio between the maximum focal distance and the minimum focal distance of the zoom lens is referred to as a zoom ratio and consumer video cameras generally use a zoom lens whose zoom ratio is about 6 to 10.
Once focused, the zoom lens has an image point that is not moved even when a zoom magnification is changed. Therefore, the zoom lens can constantly obtain a properly-focused image when a picture is taken of an object that is located at a relatively distant position under a so-called zoom-up mode.
As is well known, lenses have intrinsic minimum object distances corresponding to focal distances and therefore cannot focus an object which is located at a distance which is less than the minimum object distance.
In a zoom lens having a zoom ratio of n, as shown in FIG. 1, the minimum object distance is monotonically increased in response to a zoom magnification in a range of from a minimum object distance Dz1 in which a zoom magnification is one time (maximum wide angle lens state) to a minimum object distance Dzn in which a zoom magnification is n times (maximum telephoto lens Adf in which an object cannot be focused is defined by a curve Lz passing through the minimum object distances Dz1 and Dzn of the maximum wide angle lens state and the maximum telephoto lens state.
A minimum object distance of a zoom lens used in a consumer video camera becomes about 1 cm with the aid of a micro-closeup mechanism when a zoom lens has a zoom ratio of 8 and the zoom lens is set in the maximum wide angle lens mode in which the zoom magnification is one time, and becomes about 100 cm when the zoom lens is set in the maximum telephoto lens mode in which the zoom magnification is 8 times.
Therefore, when a picture is taken of an object which is located at the position greater than the minimum object distance of the maximum wide angle lens state and the position ahead of the minimum object distance of the maximum telephoto lens state by this zoom lens, if the zoom lens is operated from the wide angle lens mode to the telephoto lens mode in a zoom in fashion, then an image can be focused in the beginning because the zoom lens is located outside the defocusing area. Then, the zoom lens soon enters the defocusing area so that an image cannot be focused.
Furthermore, European laid-open patent publication No. 0475840 describes a lens driving apparatus of an inner focusing type in which a velocity sensor coil is wound around a coil of a voice coil type linear actuator in an overlapping fashion. According to European laid-open patent publication No. 0485302, a zoom lens portion in an inner focusing type quartet zoom lens is driven by a stepping motor. The stepping motor is driven in a micro-step driving fashion having low noise and low vibration when the zoom lens is moved at a low speed and also is driven in a high torque pulse driving fashion when the zoom lens is moved at high speed. Therefore, the driving system can be made in high speed, long in lifetime and low in noise and vibration. This previously-proposed technique is for use with a small camcorder (camera-recorder). European laid-open patent publication No. 0560549 describes a camcorder in which, when an optical viewfinder zoom and an electronic zoom are used together, a picture frame whose size is changed in response to a magnification of an electronic zoom is displayed within a viewfinder so that an actual shooting range can be accurately displayed within the viewfinder.